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Before the New Horizons spacecraft zipped past Pluto four months ago, planetary scientists weren’t sure what they were going to find at the tiny, distant world. But they did not expect to discover active volcanoes. And yet that's precisely what scientists appear to have found after combining images with topography data.

“When you see a mountain with a big hole in the top of it, that basically points to one thing,” said Oliver White, a volcano expert at NASA’s Ames Research Center. “While it's crazy, it's the least crazy idea we can think of to explain what we see.”

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White joined other scientists on the New Horizons team Monday presenting dozens of new findings at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in National Harbor, Maryland. During a news conference, scientists delved into the chaotic nature of Pluto’s system of small moons, its curious atmosphere, and other tidbits. But the likelihood of cryovolcanoes that are still remaking the surface of Pluto stole the show.

Instead of erupting molten rock like volcanoes on Earth, cryovolcanoes erupt materials such as water, ammonia, or methane. Although planetary scientists have found evidence of cryovolcanism on other worlds in the outer solar system, such as in plumes of water emanating from fissures on Saturn’s moon Enceladus, they haven’t found anything like cryovolcanoes that look like terrestrial volcanoes.

“It’s just astounding that in all of the exploration we have done in our solar system, the nearest neighbor we have found to Pluto is Mars,” said Alan Stern, principal investigator of the New Horizons mission. “You have to look all the way across the middle worlds of the solar system to find something like this.”

Scientists found two likely volcanoes in an area just south of Pluto's “heart,” which they dubbed Wright Mons and Piccard Mons. Both features are about 100 km across, and rise 3 km and 5.5 km, respectively, above Pluto’s surface. Both have distinct depressions at their summits, likely formed by collapse as ice erupted from beneath.

Enlarge/ Locations of more than 1,000 craters mapped on Pluto by NASA’s New Horizons mission indicate a wide range of surface ages.

NASA/JHUAPL/SwRI

Volcanism isn’t the only evidence that Pluto’s surface is remaking itself. Scientists also released a detailed map showing the location of more than 1,000 craters mapped on the surface during the flyby. In some areas there are lots of craters, but the heart is distinctive for its smooth surface. On the right lobe of the heart, Tombaugh Regio, there are a handful of craters, and on the left lobe, Sputnik Planum, there are none.

This variation in Pluto’s terrain allowed the scientists to estimate the age of its surface. Across the northern tier of the planet, where craters are plentiful, they estimated the age of the surface to be about 4 billion years old. In the Tombaugh Regio the age is probably 1 billion years, and the Sputnik Planum’s surface is less than 10 million years old.

Like most good science, the surprising new findings raise more questions than they answer. Scientists still don’t have a good understanding of the heat source driving Pluto’s active geology. Perhaps, they speculated, there is enough residual radioactive heat from Pluto’s silicate core to drive periodic ice eruptions, which do not require as much energy as magma eruptions.

Another question that bedevils the Pluto scientists is how the dwarf planet and its largest moon, Charon, could be so starkly different. They presumably formed at the same time, from the same environment and materials, and yet Pluto harbors a young surface, whereas nearly all of Charon is at least 4 billion years old.

Enlarge/ Pluto and its moon, Charon, are vastly different despite forming at the same time and place.

NASA/JHUAPL/SwRI

Fortunately, today’s data dump is just the beginning. By presenting their findings at the influential meeting of planetary scientists, the New Horizons mission is now bringing the larger community of researchers into the discussion. This will allow new voices and new ideas into the discussion to find explanations for Pluto’s mysteries. “This is when the debates begin,” said Curt Niebur, the NASA program scientist who oversees New Horizons. “This is when the entire science community starts to stay up throughout the night, batting ideas back and forth.”

The debates should continue for some time. New Horizons won’t even finish returning all of its data for another year.